The invention relates, generally, to control units for monitoring the emission of volatile substances in tank systems, especially of fuel vapors in motor vehicles as a consequence of a tanking operation. The invention relates especially to such a tank system wherein an adsorption filter is provided for taking up the forming volatile substances and wherein a tightness check of a tank leakage diagnostic arrangement is carried out from time to time.
In motor vehicles driven by internal combustion engines, a venting of the fuel supply tank is absolutely necessary for a continued supply of fuel. Air must flow into the tank when fuel is consumed because a vacuum would otherwise form in the tank and the fuel flow would be interrupted. The tank is, however, also to be vented in order to permit the contents of the tank sufficient opportunity for expansion during warming. In addition, enough air must exit from the tank when tanking so that the introduced fuel does not immediately splash out of the fill pipe provided on the tank.
In motor vehicles, tank-venting systems are increasingly utilized having a so-called xe2x80x9con-board refueling vapor recoveryxe2x80x9d (ORVR) function. Here, vaporizing or excessive fuel vapor or fuel gas is not directed into the ambient but is directed into an active charcoal filter (ACF) via a venting line having a large cross section. During a tanking operation, the necessary overpressure therefor adjusts because the particular pistol-like fill nozzle tightly closes against the fill stub with the aid of a rubber bellows. The fuel vapor is stored in the ACF and is, during driving operation of the motor vehicle, supplied via an electromagnetic tank-venting valve to an intake manifold of the engine and therewith to the combustion. In this way, an emission of the environmentally damaging fuel vapors from the tank is substantially prevented from reaching the ambient and, at the same time, the vapors, which are supplied to the engine, are themselves utilized as fuel whereby the fuel consumption is reduced.
In this connection, reference is made to the statutory requirements for operating internal combustion engines in regions of the United States. According to these statutory requirements, it is necessary that motor vehicles wherein volatile fuels such as gasoline are used, have a control unit of the kind mentioned initially herein which is capable to locate a leakage in the tank or in the entire fuel tank system utilizing on-board means.
In summary, tank systems of the kind mentioned initially herein have to fulfill the following tasks:
(a) a tanking operation of the fuel tank must be reliably ended when a pregiven maximum tank fill level is reached and especially while avoiding a splashing of fuel. The end of tanking is reached when the fuel in the fill pipe climbs up to the fill nozzle and then switches the latter off.
(b) during tanking, it is to be ensured that air or fuel vapor displaced in the fuel tank (in the above-mentioned ORVR tank systems) can escape from the tank into the ACF.
(c) an operational venting of the fuel tank or a pressure compensation in the tank must be guaranteed all of the time.
(d) in the case of damage to the vehicle (especially when the vehicle is turned over), a draining of the fuel is to be effectively prevented (so-called roll-over function).
(e) a tank leakage diagnosis for checking the tank system as to tightness must be possible all of the time especially during driving operation (so-called on-board diagnostic function).
From U.S. Pat. No. 5,890,474, it is known for carrying out a leakage diagnosis that first the tank system is closed off with respect to the ambient by means of an electrically drivable check valve and, thereafter, the tank system is charged with an overpressure or underpressure. A leakage diagnostic method, which is improved with respect to the above, provides, in contrast, a switch-over valve by means of which a leakage diagnosis pump can be switched over between a reference leak and the tank or the tank system.
In addition, known tank systems for satisfying the above-mentioned functions have a plurality of mechanical tanking valves. Inter alia, a float valve ensures the above-mentioned escape of displaced air during a tanking operation. When the tank fill level reaches the provided maximum elevation, then the float valve closes and thereby prevents an escape of the displaced air. As a consequence, the pressure in the tank increases which, in turn, causes the fuel in the fill pipe to climb until it reaches the level of the fill nozzle. The fill nozzle detects this liquid fuel and thereupon shuts off the flow of fuel. For this reason, measures are provided which effectively prevent a splashing of fuel also in this tanking phase.
In view of the above, it is an object of the invention to provide a tank system as well as a method for its operation which avoid the above-mentioned disadvantages of the state of the art and permit a low emission operation of such a system especially during a tanking operation with means as simple and cost-effective as possible.
The method of the invention is for operating a tank system including a tank system of a motor vehicle. The tank system includes: a tank for receiving a fluid medium forming a volatile substance; an adsorption filter connected to the tank for receiving the volatile substance which emanates from the tank; a shutoff valve operatively connected to the adsorption filter and being electrically drivable between an open position and a closed position whereat the tank system is closed off pressure tight with respect to the ambient; a tank fill-level sensor for detecting the fill level of the fluid medium in the tank and for transmitting a signal indicative of the fill level of the fluid medium; and, a fill pipe connected to the tank for filling the tank with the fluid medium. The method includes the steps of: detecting the fill level when tanking the tank; and, driving the shutoff valve into the closed position when the fill level has reached a maximum fill level.
The invention is based on the idea to replace the mechanical valves utilized up to now by the least number of electrical valves possible. Preferably, the invention provides a single electrically drivable check valve which satisfies all of the above-mentioned functions. In the case of a check valve or switch-over valve already present, such as in the initially-mentioned ORVR tank systems, this is retrofitted as an electrical tanking valve in the manner provided by the invention.
Accordingly, the invention advantageously makes unnecessary the mechanical or electrical tank valves required up to now. A check valve or switch-over valve, which is already present for tank leakage diagnosis, is used to satisfy all the functions in accordance with the invention which are necessary for tanking and operational venting. This savings leads to a cost savings as well as to a weight reduction of the motor vehicle concerned. It is noted that the optionally provided tank cap switch also affords advantages for the tank leakage diagnosis because it can, for example, prevent a diagnosis operation from starting so long as the tank cap is open.
In order to prevent still more effectively a splashing at the end of the tanking, it can be further provided that the venting cross section is reduced in a stepwise manner when closing the check valve whereby the overpressure in the tank only increases slowly. In this way, it is prevented that the fuel splashes out of the fill pipe at the end of tanking.